Canvas and DockPanel

Canvas and DockPanel sit at opposite ends of the WPF layout spectrum. Canvas is the only built-in panel that supports true absolute positioning: children are placed using the attached properties Canvas.Left, Canvas.Top, Canvas.Right, and Canvas.Bottom, and Canvas performs no automatic sizing or repositioning as its own size changes, which makes it ideal for diagramming tools, custom drawing surfaces, or game-like overlays where exact coordinates matter. DockPanel sits closer to the flexible end: children declare which edge of the remaining space they want to occupy via the DockPanel.Dock attached property (Top, Bottom, Left, or Right), and DockPanel consumes space from the outside in, shrinking the remaining area for each subsequently docked child, which makes it the natural choice for classic application shells with menus, toolbars, status bars, and a central content region.

Canvas.Left and Canvas.Top position a child relative to the Canvas's top-left origin, while Canvas.Right and Canvas.Bottom position relative to the bottom-right; if both Canvas.Left and Canvas.Right are set on the same element, Canvas.Left wins and Canvas.Right is ignored. Because Canvas gives each child essentially infinite available space during Measure, HorizontalAlignment and VerticalAlignment have no effect on Canvas children, and setting Width/Height on a child is the only way to control its size — Canvas never stretches or shrinks children to fit. Canvas also introduces a ZIndex attached property (inherited from Panel but especially relevant here) that controls paint order among overlapping children, which is essential for diagramming scenarios where connector lines need to render behind the nodes they connect.

DockPanel processes its children in the order they appear in XAML, consuming space from the edge specified by each child's DockPanel.Dock value and shrinking the remaining available rectangle for subsequent children. This means order matters: if you dock a child Top and then a second child Left, the left-docked child only occupies the space beneath the top-docked one, not the full original height. A common real-world layout docks a Menu to the top, a ToolBar beneath it (also Top), a status bar to the Bottom, and a navigation TreeView to the Left, leaving the remaining central rectangle — typically filled by the final, undocked child — as the primary content area for the actual application content.

DockPanel's LastChildFill property, true by default, determines whether the final child (the one with no explicit dock direction, or simply the last one in markup order) automatically expands to fill whatever space remains after all the docked siblings have claimed their edges. If LastChildFill is set to false, the last child instead docks to whatever direction its own DockPanel.Dock specifies, and no child automatically fills the remaining central area — which is rarely what you want, since it typically leaves a visible unfilled gap. Because dock order is determined purely by XAML declaration order rather than any explicit z-index or priority property, reordering elements in markup directly changes the resulting layout, which is a frequent source of confusion when refactoring a DockPanel-based shell.